1. Field of the Invention
The present invention relates to an electrostatic ink jet head that is provided with a micro-actuator utilizing static electricity.
2. Description of the Related Art
FIG. 1 is a perspective view of a conventional ink jet head that utilizes static electricity. FIG. 2 is a sectional view, taken along the line IIxe2x80x94II of FIG. 1, showing the structure of one actuator of the ink jet head shown in FIG. 1. In these figures, reference numeral 10 indicates an electrode substrate, reference numeral 20 indicates a liquid chamber/diaphragm substrate, and reference numeral 30 indicates a nozzle substrate. This nozzle substrate 30 is provided with a nozzle 31, and the liquid chamber/diaphragm substrate 20 is provided with ink liquid chambers 21 that communicate with the nozzle 31. A conductive diaphragm 22 is disposed as a part of the ink liquid chamber 21, and also serves as a part of a common electrode. The diaphragm 22 is thin and has a low rigidity, so as to be flexible. The electrode substrate 10 has individual electrodes 11 outside the ink liquid chambers 21 that are arranged at predetermined intervals. Reference numeral 12 indicates a protection film for preventing short-circuiting between the diaphragm 22 and the individual electrode 11. Reference numeral 13 indicates a sealing member that seals openings in which the individual electrode 11 is disposed. As shown in FIG. 1, the electrostatic ink jet head has a plurality of actuators, and each of the actuators discharges ink droplets.
In FIGS. 1 and 2, a voltage is applied between the diaphragm 22 and the individual electrode 11. The diaphragm 22 is displaced toward the individual electrode 11 due to the static electricity. Here, the applied voltage is turned off to return the diaphragm 22 to the original location at which the diaphragm 22 was situated prior to the application of the voltage. This mechanical behavior of the diaphragm 22 with respect to the static electricity is used for discharging the ink in an electrostatic ink jet apparatus. In FIG. 2, the space between the substrate 20 having the diaphragm 22 and the individual electrode 11 is normally sealed by the sealing member 13 so as to ensure isolation from the outside. This space is called a xe2x80x9cgap chamberxe2x80x9d, and the part of the gap chamber immediately below the diaphragm 22 is referred to as a diaphragm chamber.
When a voltage is applied between the diaphragm 22 and the individual electrode 11 in the electrostatic ink jet head described above, the diaphragm 22 is displaced due to static electricity that acts between the diaphragm 22 and the individual electrode 11. Therefore, the diaphragm 22 is made so thin as to reduce the driving voltage. As a result, the driving voltage can be low, but the rigidity of the diaphragm 22 becomes too low. The existence of air or gas in the diaphragm chamber or the gap chamber has an adverse influence on the behavior of the diaphragm 22. When the diaphragm 22 approaches the individual electrode 11, the diaphragm 22 is subjected to the compressive resistance of the air. As a result, the voltage at the contact point between the diaphragm 22 and the individual electrode 11 (hereinafter referred to as xe2x80x9ccontact voltagexe2x80x9d) becomes higher in a dynamic state than in a static state.
There is another problem with the conventional electrostatic ink jet head. FIGS. 3A and 3B illustrate the problem of the conventional electrostatic ink jet head. FIG. 3A shows a displacement D of the diaphragm when the driving frequency is low, and FIG. 3B shows a displacement d of the diaphragm 22 when the driving frequency is high. The diaphragm 22 of the electrostatic ink jet head (Reference numeral 22xe2x80x2 indicates the diaphragm 22 displaced and brought in contact with the individual electrode 11.) needs to be dynamically vibrated at a frequency on the order of and up to 10 kHz. The diaphragm chamber is originally small in volume, and the diaphragm 22 moves within the small space. As a result, the diaphragm 22 is subjected to the compressive resistance of the air, and the air is unlikely to return into the diaphragm chamber once it moves out of the diaphragm chamber. If the driving condition (the shape of the driving voltage pulse) is the same, the amount of air moving out of the diaphragm chamber varies with the frequency of the driving voltage pulse. The higher the frequency, the larger the amount of air that cannot return to the diaphragm chamber. As a result, the diaphragm 22 moves closer to the individual electrode 11.
FIG. 6 shows operation results of the conventional electrostatic ink jet head. As shown in FIG. 6, as the frequency becomes higher, the amount of air that cannot return to the diaphragm chamber becomes larger. As a result, the diaphragm 22 is vibrated at a location closer to the individual electrode, as shown in FIG. 3B. Accordingly, the distance between the diaphragm 22 and the individual electrode 11 actually becomes shorter, and the contact voltage becomes lower. In this manner, the frequency characteristics lead to a problem when the frequency becomes high. This phenomenon is peculiar to an electrostatic actuator that drives a diaphragm by static electricity, and should be eliminated when a high-frequency driving operation is carried out.
The above problem arises only when the contact driving operation is performed, with the diaphragm being in contact with the electrodes. In a non-contact driving operation, the above problem of frequency dependence is not caused or can be neglected.
As described before, the diaphragm is subjected to the compressive resistance of the air in the gap chamber in the conventional electrostatic ink jet head. As a result, there will be a problem that the contact voltage increases. To solve this problem, there have been several suggestions. For instance, Japanese Laid-Open Patent Application No. 7-299908 discloses an electrostatic ink jet head in which a space for the air, as well as the diaphragm chamber, is formed in the gap chamber, so that the diaphragm displaced toward the electrodes is not subjected to the compressive resistance of the air. This will result in a larger gap chamber.
However, there has been no suggestion as to a method to solve the problem that arises in a high-frequency driving operation. This is because such a problem is unlikely caused in a conventional electrostatic ink jet head having the maximum driving frequency of 10 kHz, for instance.
It is a general object of the present invention to provide electrostatic ink jet heads in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an electrostatic ink jet head in which the volume of the diaphragm chamber is relative to the volume of the gap chamber, and the volume of the gap chamber except the diaphragm chamber can be smaller than that in the prior art.
Further specific objects of the present invention are: to improve the frequency dependence of the electrostatic actuator simply by setting the waveform of the driving voltage; to improve the frequency dependence of the electrostatic actuator having a certain gap configuration; to improve the frequency dependence of the electrostatic actuator by changing the structure and configuration; and to improve the frequency dependence of the electrostatic actuator both by changing the structure and configuration and by setting the waveform of the driving voltage.
The above objects of the present invention are achieved by an electrostatic ink jet head that comprises a diaphragm, and an electrode that faces the diaphragm, with a predetermined gap chamber being maintained between the electrode and the diaphragm. In this electrostatic ink jet head, a pulse voltage is applied between the electrode and the diaphragm so as to deform the diaphragm by static electricity. Ink droplets are discharged by a mechanical recovering force of the deformed diaphragm. In this electrostatic ink jet head, one pixel is formed with a pulse voltage. The period of time in which the diaphragm is in contact with the electrode is 40% or less of the period of time required for forming one pixel.
With the electrostatic ink jet head of the present invention, the proportion of the pulse voltage to be applied between the diaphragm and the individual electrode (i.e., the period of time during which the diaphragm is in contact with the electrode) to the period of time required for forming one pixel can be suitably selected. Thus, the frequency characteristics can be greatly improved, and the ink discharging characteristics can be stabilized. Accordingly, the reliability of the electrostatic ink jet head can be increased.
In the electrostatic ink jet head of the present invention, one pixel may be formed with a plurality of pulse voltages.
Also, the electrostatic ink jet head of the present invention may include a plurality of electrostatic actuators. Each of the plurality of electrostatic actuators comprises: a nozzle; an ink liquid chamber that communicates with the nozzle; a diaphragm that is a part of the ink liquid chamber and a part of a common electrode; and an individual electrode that faces the diaphragm and is disposed outside the ink liquid chamber, with a predetermined gap being maintained between the individual electrode and the diaphragm. A pulse voltage is applied between the diaphragm and the individual electrode so as to deform the diaphragm by static electricity, and ink droplets are discharged through the nozzle by a mechanical recovering force generated in the deformed diaphragm. The period of time during which the diaphragm is in contact with the individual electrode is 40% or less of the period of time required for forming one pixel.
The above objects of the present invention are also achieved by an electrostatic ink jet head that comprises a diaphragm, and an electrode that faces the diaphragm, with a predetermined gap being maintained between the electrode and the diaphragm. In this ink jet head, a pulse voltage is applied between the electrode and the diaphragm so as to deform the diaphragm, and ink droplets are discharged by a mechanical recovering force of the deformed diaphragm. Where the volume of the gap chamber is V, and the volume of a diaphragm chamber that is a part of the gap chamber and formed by a space between the diaphragm and the electrode is V1, the relationship, V1/V greater than 0.7, is satisfied.
With the electrostatic ink jet head of the present invention, the ratio of the volume of the diaphragm chamber to the gap chamber can be suitably selected. Thus, the frequency characteristics of the head can be greatly improved, and the ink discharging characteristics can be stabilized. Accordingly, the reliability of the ink jet head can be increased.
The above objects of the present invention are also achieved by an ink jet recording apparatus on which the any one of the above electrostatic ink jet heads is mounted. In this ink jet recording apparatus, the electrostatic ink jet head faces a recording sheet, and discharges ink droplets while reciprocating with respect to the recording sheet, thereby performing a recording operation.
Other objects and further features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.